A variety of methods like electrodialysis (ED), pervaporation (PV), solvent extraction (SE), esterification, vacuum distillation, etc. are reported for separation of acidic components from a solution in the literature.
U.S. Pat. Nos. 5,786,185; 5,968,362; 6,419,828; European patent EP0265409 disclose a method of recovery of acids using solid or liquid anion exchanger in presence of water or a suitable organic solvent in which the acid binds to the ion exchanger and is removed from solution containing other non-acidic component or impurities and the bound acid is eluted by a suitable solution or by a solvent.
Another method of purification of acids is by extraction with solvents that selectively or preferentially remove the acid and is disclosed in U.S. Pat. Nos. 5,089,664; 6,087,532 and 5,959,144.
U.S. Pat. No. 4,885,247 discloses electrodialysis method for removal of ionic compounds like salts of different acids using the cationic and anionic membranes with electrical charge. Bipolar electrodialysis method is also reported for the recovery of organic acids from their solutions (U.S. Pat. Nos. 6,280,985; 6,319,382 B1; 6,294,066).
U.S. Pat. No. 6,534,679 discloses a process for purification of organic acids by their esterification with suitable alcohol, distillation of the ester followed by hydrolysis of the ester to get pure acid.
Pervaporation using suitable membranes is also used for the removal or concentration of volatile organic and inorganic acids (Korean Patent KR 9511091B1; German Patent DE 4409906 C1; U.S. Pat. No. 4,892,661 A).
U.S. Pat. No. 6,489,508 B1 and Chinese patent 1335294 disclose a method of removal of organic compounds including acids, such as lactic acids by distillation using a high vacuum in a short path distillation mode.
Several chemical methods for removal of acids from complex mixtures by either extraction or ion exchange are being replaced by physical processes like membrane processes and distillation due to the increased environmental concerns.
The drawbacks of the existing processes for removal of organic acid from fermentation broth are given below:
The conventional processes for recovery of acid have several limitations such as need of multi-step process to recover acid, high energy requirement, generation of huge quantities of insoluble inorganic salts, process limitations (imposed by physical properties like boiling point, partition coefficient, etc), rate of separation and selectivity.
In the conventional process of recovery of lactic and citric acid from fermentation broth using calcium salt, the generation of huge quantities of insoluble calcium sulphate is a major problem.
In an ion exchange method using solid ion exchangers, there is a need for use of chemicals for regeneration of ion-exchange resins, giving rise to waste streams. It also causes undesirable dilution of the product stream. In addition, feed cannot be directly fed to the system and needs clarification and cleaning. In case of use of liquid amines like trioctyl amine as extracting agent, the regeneration of the extractant needs alkali or heat and can lead to loss of the amine. In addition, the un-recovered amines in the waste stream can be toxic and become environmental problem. The relatively high cost of such amines is also an obstacle in using them at industrial levels.
In case of deacidification or concentration of acids by electrodialysis, the feed needs to be treated prior to charging in electrodialysis cell. The capacity of electrodialysis cell also decreases due to deposition of Ca++ or Mg++ ions on the membrane. In some of the cases, particularly during fermentative production of organic acids, where bases are used for pH maintenance, the possibility of cell culture recirculation back to the fermentor reduces, as the death possibility increases. In many literature reports on electrodialysis, the transport of water, sugars and proteins from feed across the membrane are also well documented. Hence, the transported acid also contains some of the unwanted impurities. More importantly, the requirement of high amount of electrical power for electrodialysis-based separation is a major draw back of this process.
In case of conventional neutralization, salt formation could be disadvantageous, as it remains in the solution. Such salts might create purification and processing issues and can lead to the environmental pollution. Further, this method does not allow the reuse of acid.
In the process of distillation using short range and high vacuum, several unwanted volatile compounds get distilled and contribute to undesired impurities in the distillate that might subsequently become difficult to remove.